A background in Engineering, Facilities management and a solid understanding of mechanical systems, HVAC, fire alarms and life safety are the basic requirements.
I carry a number of licenses, am HVAC certified and factory trained, have over 30 years in the industry and still learn new things daily.
The industry is leaning towards "Green: services which equates to energy conservation, non volatile cleaning and paint supplies and many other things.
I would suggest you have at least:
Basic administration skills
Basic HR knowledge
Life safety support systems
Pools and spas
Electrical systems and lighting
Motor control systems
interior and exterior finishes
general landscape for the region
Got the idea, it takes decades to make the Grade.
Preventive Maintenance programs